In a world where time travel is possible, but has not been invented yet, would travelers from the future come back to prove it exists?

Surely, they would bring that technology back in time to let the people of the past use it?

I have four theoretical explanations so far, can anyone explain them in more detail, or provide other possibilities?

The people decided to ban backwards time travel to prevent mass panic in the past, or to prevent possible paradoxes.

The people in the future went back in time to an alternate past. (Similar to the Many worlds interpretation but with time travel).

The people come back in time to give the people in the past new technology, they then jump forward in time to see how humanity has progressed using this. The result is that the future they return to is different to the one they left.

They cannot go back in time because it has already happened, and therefore they can only go forward in time.

$\begingroup$Time travel isn't possible in worlds; it's a feature of the Universe (where "Universe" is what we used to call the Multiverse). I know that sounds picky, but it's an important concept to keep in mind.$\endgroup$
– RonJohnMay 4 '17 at 15:05

40 Answers
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In all probability, we, 21st century barbarians, will not be very well loved by our 23rd century descendants.

Non-historians of that age will probably group us together with the generations of the late 20th century, and refer to us collectively in their high-school level history courses. We are the generations who baked the earth and poisoned the seas. We wiped out thousands of animal species and held the world hostage for almost a century under the threat of nuclear or biological annihilation. Our insanity almost brought about human extinction and led to the Second Dark Ages, from which they are only just recovering.

"You want to give our time travel technology to those mad men? ARE YOU NUTS? Leave them in the past where their damage has already been done. We don't need them coming up here to further screw up our present!"

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– Monica CellioAug 14 '16 at 4:56

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$\begingroup$Why would travelling back be the same as giving them (us) the technology? If their (our) behaviour was (is) so bad, chances are we (they) want to prevent it happening in the first place. You could go back and kill Mao, Hitler, Stalin or any other serial killer there was or even ALL of them - and you don't do that because ... they were evil?$\endgroup$
– nullAug 18 '16 at 14:12

$\begingroup$Anybody intelligent enough to invent time travel is also going to be intelligent enough to know that cause and effect exists. While "going back in time to kill Hitler" is a great trope, knowledge of cause and effect would lead to the decision that doing so is a terrible idea. Killing Hitler causes more harm than good, and the inventor of time travel would know that.$\endgroup$
– Space OstrichSep 27 '16 at 7:30

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$\begingroup$I disagree with the premise of your answer.$\endgroup$
– Xandar The ZenonDec 24 '16 at 5:50

$\begingroup$This for some reason assumes that humanity will be more reasonable in the future. Spoiler: It won't.$\endgroup$
– GloweyeOct 16 '19 at 8:28

In the film Primer, there was a time travel machine that could only jump as far back as when it was first turned on. This puts a practical limit to how far back you can jump to the point where time travel was invented in the first place.

$\begingroup$Interesting - this could be explained in some way by quantum entanglement perhaps. That the time machine works by principles of quantum mechanics, and the initial entangling anchors it in time and space to it's creation.$\endgroup$
– Jesse WilliamsAug 11 '16 at 1:34

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$\begingroup$This is the way wormholes are thought to work. Due to time dilation wormholes can be used to travel through time, but only to the specific point in time in which the other end currently is. Obviously, this means that no end of the wormhole will ever be before the moment at which the wormhole was first created. Or after the one at which it was destroyed/otherwise ceased to exist. Of course that is if wormholes are possible at all, which is still unclear.$\endgroup$
– AnnonymusAug 11 '16 at 5:53

Because the earth orbits the sun at 30km a second, the solar system is moving at 20km a second and the galaxy is moving at 230km a second! Any time travel will result in the people being dumped out into the void of space...

Maybe we need to set up some beacons to guide people to the earth through time and we can only set these up now...

$\begingroup$Comments are not for extended discussion; this conversation has been moved to chat.$\endgroup$
– Monica CellioAug 12 '16 at 1:46

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$\begingroup$The question obviously implies that this minor inconvenience will be solved by an advanced enough future species that is able to do time travling...$\endgroup$
– AnoEAug 12 '16 at 14:01

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$\begingroup$This is not a sound argument without further explanation. Why do you assume that the time machine works on positions in the galaxy's, or cluster's, reference frame? This sounds like a nonrelativistic view, similar to the aether, which is now a fringe theory practically nobody believes in. If anything, one could complain that a position on Earth is not an inertial system, due to the involved rotations.$\endgroup$
– VandroiyAug 12 '16 at 17:45

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$\begingroup$This answer is really intriguing. It highlights the spatial aspect of time travel: if you go back in time wanting to appear at point X on the earth's surface, you'll need to calculate exactly where point X would be at that point in time. Doing this with high precision isn't trivial. Over long time-scales (millions of years) the planetary orbits of our solar system are chaotic, meaning it would be impossible to compute precise orbital positions in the distant past. So to answer the original question, you could posit a time travel mechanism that can't work without high spatial precision.$\endgroup$
– Jarrod SmithAug 15 '16 at 4:40

$\begingroup$Since the universe is expanding, the earth will never be in exactly the same place twice.....$\endgroup$
– AricAug 15 '16 at 8:30

Here's another option that is often forgotten: Time travel might need an exit system available at the destination. Since we don't have an exit system in our time, time travellers cannot visit us.

For example, time travel might be done through a wormhole whose two ends are shifted in time relative to each other, so if you enter one opening, you come back through the other opening, say, one year earlier. Going back several years would work by passing it several times, and going back is done by passing it the opposite way. Then of course you only can travel to times after that wormhole has been established.

Edit:

Here's a second thought that occurred to me: What if the time machine doesn't travel with you? Think of the Star Trek transporter: If people are beamed to the surface of a planet, the transporter doesn't travel with them; it remains on the ship. If they couldn't ask the people on the ship to beam them up again (say, if the transporter could only beam them away), and there were no other means to fetch them back (like shuttles), they would be stranded on the planet.

In a similar way, a time traveller that comes to our time would be stranded in our time; we simply don't have time machines to send him back into his time, and probably don't have the ability to build one in his life time even if they'd bring the plans with them, nor the energy required to operate it. Which would be a strong disincentive for anyone to visit us, unless he actually wants to live in our time — but in the latter case, he'll probably not tell us that he's a time traveller, as that will be counter productive for his plans.

$\begingroup$Your first argument is good, but the second one has a problem; it only takes one outlier to ruin it all. While it would be a great disincentive for most, in the near infinity of humans who will exist in the future there's bound to be at least one who doesn't care about not being able to return or remaining hidden. Even among those who try to hide there could be somebody who simply makes an error. Or somebody who tries to hide something that is difficult to hide. Maybe they brought their FeelGood(tm) entertainment console thinking "it's ok if I hide it" even though it is against the rules.$\endgroup$
– AnnonymusAug 11 '16 at 6:01

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$\begingroup$@Annonymus: And what is the probability that this single outlier exactly hits our time? I mean, there are millennia past us, and from the point of view of that future person, we are only slightly less primitive than the Romans were. Maybe the Antikythera mechanism was actually built by a time traveller. Maybe it was a time traveller that sparked Greek philosophy. Maybe Jesus was a time traveller. Maybe John Titor was real.$\endgroup$
– celtschkAug 11 '16 at 7:45

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$\begingroup$Your first explanation actually matches up with how things would work in theoretical time travel schemes thought up by physicists, like time travel via traversable wormholes--according to how wormholes would theoretically work in relativity, it would be impossible to travel back any further than when the time differential was first created between the two wormhole mouths (which would have to be done by taking one wormhole mouth on a trip at some significant fraction of the speed of light relative to the other one).$\endgroup$
– HypnosiflAug 11 '16 at 10:08

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$\begingroup$@Annonymus: there are dozens of those people amongst us today. We call them "crackpots" and lock them up. If you come here without a time machine, a bunch of talk of the future that can't be confirmed and try to convince people that you're a real time traveller, you're just going to end up in an asylum.$\endgroup$
– ErikAug 11 '16 at 16:49

$\begingroup$If you are Lex Luthor from 2200 and escape to the past hiding from the cops, you won't want to be telling that you are a time traveller so you can be detected. You will hide that fact and pretend to be a normal guy born on this age, you just took enough gadgets to take over some corporations...$\endgroup$
– ÁngelAug 11 '16 at 22:23

They are able to travel to the past, but are unable to alter it. This includes showing off their time machines or other future technology or knowledge, or anything else that might remotely change the progression of events. This is the Novikov self-consistency principle.

They have come back and told us, but their influence is retroactively expunged by other time-travellers charged with maintaining a consistent past.

@chrisj hit on the grand problem of time travel: you have to travel in time and space. I'd love to expand on all the ways this ties time travel to space travel.

The surface of the Earth is rotating at 0 to 465 m/s depending on your latitude. Fortunately momentum will be conserved (it must otherwise that would imply there is an absolute rest state for the universe) so when you jump you will also be traveling at that speed. However you'll be somewhere else. Even jumping 1 second back will place you 465 meters away... possibly in a brick wall. This is the least of your problems, because this assumes the Earth is standing still.

The Earth is moving around the Sun at about 30,000 m/s. Now if you jump back in time 1 second you'll be 30km away from where you started, probably either 30km inside the Earth or 30km in the air. Not good, but with some careful calculations you could construct a plane or spaceship or something? And if you want to jump back years you just make sure to jump when the Earth is in the same spot in its orbit. I have bad news.

The Solar System is rotating around the center of the galaxy at 220,000 m/s taking the Earth with it. Now if you jump back one second you'll be somewhere between 240 km and 200 km away from where you jumped, depending on which way the Earth was moving in its orbit. That's bad, but a spaceship can still fix that, right?

Let's say time travel is invented in 2216 and they want to come back to 2016 to tell us all about it! In that time the Solar System will have moved...

That's ok, your time machine is also a spaceship! You can fly back to the Earth. And you're in front of the Earth, so it will catch up to you! Well, I have some more bad news.

You came out in front of the 2016 Earth, yes, but you are continuing to move at 220 km/s around the center of the galaxy in the same direction as the 2016 Earth (the difference in angle is so slight it does not matter for rough calculations). The Earth will never catch up to you. You're going to have to fly all 0.147 ly back to the Earth.

What if you went even further back in time to give yourself more time to travel back to the Earth... now you pop out even further away from the Earth and have to travel even further to get back to it...

Basically your time machine also needs to be an interstellar space ship. Maybe time travelers have visited us.

P.S. There is a version of time travel where this is not a problem. Instead of jumping from one point in time to another, you instead speed up time. Now you are passing through every point in time. This means you are subject to the forces of gravity and held in place on the Earth.

$\begingroup$... I must say, out of all possible explanations, this one depresses me the most...$\endgroup$
– TaegostAug 10 '16 at 18:42

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$\begingroup$There is no preferred reference frame. It moved according to which reference frame?$\endgroup$
– JDługoszAug 10 '16 at 19:30

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$\begingroup$This idea about time travel was first used by Terry Carr in a short story "Brown Robert", Magazine of Fantasy & Science Fiction, July 1962. He expected it revolutionize time travel stories or stop them in their tracks. It didn't. Writers ignored it & kept on writing time travel stories the way they always had done. It only applies to time travel where there is a pure shift in time without any spatial motion. Which seems unlikely.$\endgroup$
– a4androidAug 11 '16 at 1:11

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$\begingroup$You're missing JDługosz's point. The entire idea of this post is nonrelativistic! Why would the galaxy's reference frame be better than any other? If anything, you could complain that a position on Earth is not a real inertial system, due to the involved rotations. In any case, if you see gravity as something that curves space-time and all inertial systems as equally valid, expecting that time machines naturally care about the galactic center is absurd.$\endgroup$
– VandroiyAug 12 '16 at 17:38

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$\begingroup$@Schwern That's not what a frame of reference is. If you use the Earth's position as your frame of reference, the Earth never moves in your given frame of reference. This whole construct is not a valid argument unless you define a universal way to not move. But that sounds like a Newtonian world with an aether, which doesn't seem to be the world we live in.$\endgroup$
– VandroiyAug 12 '16 at 18:10

Ever noticed how people in time travel stories mess everything up? And then somehow by the skin of their teeth manage to put everything back to rights? Things probably don't actually work like that. About half of the time the calvary doesn't arrive in time, the answer to the riddle fails to pop into the hero's head, the match doesn't light, the green wire blows up the bomb. Time travel gives people infinite chances to mess everything up. Each chance to destroy the world plays out instantly as time travel is invented -- time travel takes all the possible spins of Russian Roulette and plays them at once.

The moment you invent time travel you fork a possible universe off, do your messy business, destroy everything and the only possible universe that still exists is the one where you didn't.

In any universes where time travel is invented the world is destroyed instantly. The fact that our world still exists can be taken as proof that we're on one of the few possible timelines where time travel doesn't, will not or cannot exist.

$\begingroup$Not to imply this must always happen in reality(!), but as an in-universe rule chosen to explain away time travel, it's pretty great. +1 ...and hey, maybe it does explain our own lack of received time-travellers ;-)$\endgroup$
– underscore_dAug 11 '16 at 14:14

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$\begingroup$An alternative to this idea is that time travel is single use. Deep in a bunker on a ruined world a team of scientists invent time travel. They return to the Cuban missile crisis, prevent nuclear war thus creating a non-ruined world, one with no bunker, a world where the scientists in question work on things like particle physics and normal crap.$\endgroup$
– jorfusMar 9 '18 at 23:27

$\begingroup$If you drive to a place where there is already a car, you'd be in a car accident.$\endgroup$
– Chris SchneiderAug 11 '16 at 16:12

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$\begingroup$Ehi TrEs-2b I have a SE account I wanted you to know that :)$\endgroup$
– CoffeDeveloperAug 12 '16 at 14:13

$\begingroup$No, if time is far enough to infinitely, then they must always has some bad guy break the law to travel back and leave a high technology trace. And the technology can be advanced enough to tell us all the story.$\endgroup$
– FruitOct 21 '18 at 15:50

Let's suppose time travel allows you meaningfully alter the past. By jumping back in time you're erasing the future you came from, leading to the good old fashioned grandfather paradox. Let's just sidestep that - You're physically in the past, so let's just assume you're now part of the past.

Kill Hitler, save Lenin and Franz Ferdinand, do whatever you want. You've now altered the time-stream such that the invention of time travel most likely happens at a different moment, if it happens at all. If the new inventors of time travel jump back the same thing will happen, either erasing you if they jumped earlier or erasing their own discovery if they jumped in after you.

If one of these jumps alters the timeline so that time travel isn't discovered, then it'll stay that way forever. If anyone from anytime can travel to any point in the past, then there's effectively an infinite chance this has happened. In the same way that people say it's statistically certain we live in a simulation, I say it's statistically certain we live in a universe were time-travel was permanently un-invented.

Option 2: Time travel is for forensics and archaeology

Maybe time travel doesn't even send you back in time - Maybe it just lets you see the past exactly as it was, like a kind of reverse Minority Report. I don't really have much to say about this one - It avoids all the problems of causality because it's non-interactive, but aside from perfect reconstruction of the past it's pretty dull. Great for documentaries, bad for trying to hunt dinosaurs.

$\begingroup$Third option, a +2,5℃ global warming sends definitely back mankind to the neolithic age after a long agony of 3 three century of civil war. So many technologies will never be discovered ever, including time travel.$\endgroup$
– user2284570Aug 14 '16 at 18:38

$\begingroup$@user2284570 Conspiracy mode: In 2773 a radical religious sect seeks to destroy earths atmospheric control nanites and return Mother Gaia to the natural order. Their plan: To travel to the past, and sew the seeds of war and strife to prevent a parasitic mankind from controlling nature.$\endgroup$
– UIDAlexDAug 15 '16 at 13:34

$\begingroup$Yes, but for my case, the first of those centuries civil wars affecting worldwide had already started (with 3 previous ones that only affect the local area). So it’s not purely fictional. Though I agree the root cause is hidden for those who don’t read ᴘᴀᴍ’s reports. And after re‑reading the question, I found that kind of answer will be out of scope.$\endgroup$
– user2284570Aug 16 '16 at 23:13

I think there's something subtle missing from the "how do you account for Earth's motion" explanation: which reference frame does your time machine use? Sure, the Earth is rotating, but that's relative to the Earth's center. It's also orbiting the sun, but that's relative to the sun. That's also orbiting the galactic center, but that's relative to the galaxy. The galaxy is moving through space relative to other galaxies or relative to the CMB.

So which reference frame does your time machine move relative to? How does it establish that reference frame and track it through time? Reference frames are sort of arbitrary, you need to tie them to something physical. And maybe that's how you resolve the problem. Maybe your time machine isn't a vehicle, it's a portal and a portal needs two ends. Only the two ends aren't separated by space, they're separated by time (or both).

So why aren't time travelers coming in from the future? Because we haven't built the time machine yet.

$\begingroup$I was about to give a similar answer. This is a theoretically plausible method, assuming that it's possible to create stable portable wormholes and that it's possible to survive a trip through one. Establish a wormhole with one end in your lab and the other end at the event horizon of a black hole. Let it sit for a fixed amount of time (say 10 years), then finally move the black hole end into your lab beside the other end. Going through one way sends you 10 years into the future, and other sends you 10 years into the past, or into a black hole if you try to go too far back.$\endgroup$
– Josh DavisAug 10 '16 at 18:43

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$\begingroup$This answer is more of a comment..?$\endgroup$
– kwahAug 10 '16 at 20:15

$\begingroup$"So why aren't time travelers coming in from the future? Because we haven't built the time machine yet." I have seen this explanation several times, and i find it quite interesting. The idea of only being ablt to go back to when the portal was built is a good one.$\endgroup$
– AricAug 11 '16 at 15:29

$\begingroup$See the difference between inertial and non-inertial reference frames. Consider how the distinction changes physics calculations. Hand-waving with "you can have any reference frame you want" doesn't solve the problem.$\endgroup$
– galdreAug 18 '16 at 14:38

Just as space expands, the dimension of time expands as well, the process of which we perceive as the passing of time. We are at the very edge of time, any new moment that happens hasn't existed before it happened. History is the coordinates of time we leave behind.

As the progression of time only happens because of the "pressure" that the past exerts on time, everytime you travel backwards in time, you collapse the present; once you travel back, the time you have travelled too is the new edge of time (so, no travelling back to the future).
[For added weirdness: Maybe the collapse only happens locally. When you travel back, you only reset those parts that you have an effect on, causing this part of spacetime to be out of synch with the rest of the universe].

There are no travelers from the future, because the future has yet to happen.

$\begingroup$You're saying "time traveling is impossible", but that's not relevant to the question. The question is postulating "time traveling is possible => why XXX". The fact that time traveling is possible is not debatable for this world-building question.$\endgroup$
– AnoEAug 12 '16 at 14:37

$\begingroup$@AnoE I'm saying that time travel is only possible in one direction, from our point of view (the "edge of time"), and any direction from any other point. In Addition, the first paragraph answers the question, the second paragraph is only meant to provide additional flavour text. Sorry if that wasn't clear from the formating.$\endgroup$
– TheSexyMenhirAug 12 '16 at 14:56

$\begingroup$I kind of understand your words, but it is hard to follow what you mean. "The future has yet to happen" - does not compute. :)$\endgroup$
– AnoEAug 12 '16 at 15:01

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$\begingroup$@AnoE : The question was created using a given assumption that time traveling is possible. However, TheSexyMenhir came up with a brilliant in-universe explanation why time travel involving the future may be impossible, even if time travel (to the past) is impossible.$\endgroup$
– TOOGAMAug 16 '16 at 2:46

$\begingroup$"time travel involving the future may be impossible, even if time travel (to the past) is impossible" there may be one im too many in there. ;)$\endgroup$
– AnoEAug 16 '16 at 6:37

Time travel to the past requires both a sending device in the future and a receiving device in the past. Without both in place, no backwards time travel is possible. It could be that no stable wormhole will form, or attempting travel without a receiver will always result in the traveler being atomized. No matter how far technology advances, nothing is able to overcome the need for a receiving device. Thus the earliest time period to which a time traveler can travel will be the first successful invention of the time machine, which, of course, hasn't happened yet.

This means that whoever invents the first working receiver will be inundated by travelers from innumerable points in the future who've come to witness the birth of time travel, and/or the earliest possible era they can travel to.

So, if you invent a time machine, make sure you have a lot of receivers built before switching any of them on. Or don't ever tell anyone or record anywhere that you made one.

A couple alternate explanations:

Only a few attempts at backwards time travel were ever attempted throughout all of (future) history. Some were successful, but for one reason or another they always seemed like catastrophic failures from the sender's perspective. No one ever returned or sent back data, and no one managed to leave a clear indicator to the future that they had succeeded. So time travel was determined to be impossible, and every few thousand years when some nut-job tried again, the same basic result occurred, keeping time travel in the "impossible" category.

Each trip back in time actually creates an alternate universe, with the original universe continuing to exist. From the original universe's perspective, time travel is unsuccessful and is instead a way of destroying matter and energy. We are in the original universe, so no time travelers have arrived in our universe because they actually arrive in an alternate one. The alternate universe's history is identical to ours until the moment a time traveler arrives in it, and which point the events begin to diverge.

$\begingroup$One conceit that has appeared in a few works is that the receiver and the sender are actually the exact same device, so no one can use a particular time machine to travel back to a time before the machine was first activated.$\endgroup$
– Doug WarrenAug 10 '16 at 20:47

Backward time travel creates a parallel timeline where it happened, the prime timeline remaining untouched.

This prevents paradoxes, it also means that the primary timeline can never have examples of backward time travel. It also means that you can never go home, never undo the damage you've done, only get slowly further and further away from the world you knew.

Another popular option being

Time travel is only possible as far back as the first implementation of the technology.

The best known of these being under the wormhole theory. Which allows you to create wormholes for time travel but you can't go back to a point before you created the first wormhole.

There's also the classic

It happens all the time, but my mate says the government covers it up.

The one remotely plausible mechanism for time travel is through "frame dragging" with a massive rotating object. The purpose built device is known as a "T machine" after Frank Tippler, who first proposed it, but it may be possible to have a similar effect using a rapidly rotating neutron star or black hole.

Essentially what happens is the rotating object bends space-time in such a fashion that light cones are "tipped" and point to "elsewhen", outside of the 45 degree boundary defined by the speed of light. The illustration demonstrates how this is done:

There are a number of different assumptions you can make (and since no one has a working T machine to test it on, many of these ideas are equally plausible).

Number one: you cannot go any farther back in time than the creation of the T machine in the first place. Since we are talking about an artificially maintained cylinder of neutronium rotating at nearly the speed of light, this isn't a project that is going to be carried out anytime in the near future (or at least our near future).

Number two: moving into the past or future creates a new "timeline", and this timeline simply sees you start spiralling around the T machine and vanishing, never to return.

Number three: Since travelling in time is essentially travelling to "elsewhen", you have left our universe. This is related to number two, since there is no guarantee that the tipped light cones will actually be pointing back to your starting point. Even in the unlikely event that your light cone is pointed back to our universe, it might not be pointed back to our region of space, or to our current time. Popping back in existence in our universe one billion years in the future or one billion light years away has some obvious problems for the time traveller.

Finally, most of these ideas are based on the assumption that you can equally access the T machine after travelling through to "otherwhen". Passing through the field of tipped light cones and emerging in an empty universe where there is no corresponding T machine is going to be a huge problem.

$\begingroup$Tipler's idea is outdated, it's been proven that in the theory of relativity it wouldn't work unless the cylinder were literally infinite in length--see here. But alternate theoeretical methods of time travel have been found in relativity which could work in a finite region, like traversable wormholes--you can read more about this in books written by physicists for a general audience, like "Black Holes and Time Warps" by Kip Thorne or "Time Travel and Warp Drives" by Everett and Roman.$\endgroup$
– HypnosiflAug 11 '16 at 10:12

$\begingroup$i did not ask for how time travel might work, considering that my first sentence in the question starts with "In a world where time travel is possible..."$\endgroup$
– AricAug 11 '16 at 10:13

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$\begingroup$@Aric Fowler - But the method may determine the answer, see my comment on celtschk's answer above.$\endgroup$
– HypnosiflAug 11 '16 at 10:14

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$\begingroup$Point taken, this could provide some insight into the answer$\endgroup$
– AricAug 11 '16 at 10:16

An option that has not been mentioned in the main answers (only found in the last one by colmde, when I thought it would not have been raised) is that such time travel discovered in the far distant future may not allow to travel back in time to where we are now. Probably not due to physics laws if it is able to travel back in time, but due to economics.

So, traveling back a week, is affordable to almost anyone.

Traveling back a month, would be a severe hit to your economy but generally doable.

Traveling back some years is something only governments or big corporations can afford.

Traveling back hundreds of years, you would need the energy of dozens of suns to power your travel machine, thus it is "impossible" even though time travel is allowed and routinely performed in this universe.

Actually, it is uncommon that it would cost the same to travel back 1 day than 100 years (but I don't know the details of the time machine that is discovered in that distant future). Moreover, the power needed to travel back probably isn't linear to the time traveled but likely quadratical.

Because you can't interact with time, in the general sense. Time travel isn't like going to Disneyland and stading in line for all the cool rides, arguing with the guy that cut in front of you, or sweating future sweat all over the past.

It's more like going to an animated wax museum and watching the same script play out over and over and over again - you can change perspective, pay attention to this or that, but you can't change any of it.

Future time travelers use the technology to study the past for what it was and not how it is portrayed by biased writers. In this way, future time travelers are able to help future soceities learn from the past's mistakes and truly make the future a better place.

If altering the past is possible then the means to do so will never be discovered.

The thing is, if time travel is possible someday someone will tamper with the past. Somebody else will tamper with the past and so on. The only stable state is the one in which the time machine is never discovered. Thus any observer in a universe with a time machine gets reset out of existence and thus all observers see no time machine.

Note that this does not preclude what I call time roads--something where the time travel technology doesn't travel in time and thus you can only reach a point while it already existed. Such a device can't be reset out of existence. (Example, Robert L. Forward, Timemaster--time travel was attained by relatavistically altering the relative age of the ends of a wormhole. Take one end to the stars and bring it back, hop through and you move a fixed distance through time.)

After inventing this technology, they tested it by going back to a point that is still in our future, activated some paradoxes, and wiped themselves out before they could send the information, people, etc. farther back.

Alternatively, they did figure out how to send information back in time, but they used quantum entanglement and other such effects to do it, so we have to wait until we've built the receivers for those messages before we notice. Analogously, we can send a radio signal deep into a forest, but if there's nobody in there with an appropriately constructed receiver, they won't notice the information being sent to them.

$\begingroup$"activated some paradoxes, and wiped themselves out before they could send the information, people, etc. farther back." Thereby causing another paradox! If they never sent the stuff back, then they couldn't wipe themselves out, so they could send the stuff back, then they'd wipe themselves out, and is it just me or is anyone else feeling dizzy. AFAICT this paradox is usually invoked to ensure things do happen (albeit without explaining how it 'first' happened), not don't.$\endgroup$
– underscore_dAug 15 '16 at 11:52

$\begingroup$@underscore_d Yep, and by the time you stop spinning and the loop breaks, the story's over. For less confusion, the wipeout event triggered by the paradox could have its effect after the moment they sent someone back ("we started experimenting with time travel, and then strange things started happening, and then this...") but before they can run another test.$\endgroup$
– WBTAug 15 '16 at 12:44

$\begingroup$Basically, one who can travel in time becomes (a) god, and there can be only one.$\endgroup$
– MichaelAug 11 '16 at 19:56

$\begingroup$In using time travel to get rid of the people you don't want to have access to time travel, it's also perfectly plausible that you rid yourself of the motivation behind inventing time travel in the first place, resulting in time travel never being invented, resulting in those people persisting, ... These time travel paradoxes give me headaches.$\endgroup$
– a CVnAug 18 '16 at 13:17

It will not be we who reach Alpha Centauri and the other nearby stars.
It will be a species very like us, but with more of our strengths and
fewer of our weaknesses, more confident, farseeing, capable, and
prudent.

I think the same concept applies to time travel. We're quickly approaching the singularity and as such, people will either become exceedingly enlightened and abandon much of what once made them such petty creatures or they will cease to exist.

The "humans" (more likely to be cyborgs) of this future will understand the long-reaching effects of their actions and won't want to interfere with the past if it might cause unforeseeable consequences to the future.

2.) Humans of the future have already been wiped out

It can't be said that humans have passed the Great Filter yet. We're still living in tumultuous times. We still might wipe ourselves out. No humans in the future means no one to come visit us.

3.) Alternate time lines

As others have mentioned, it may be the case that alternate timelines exist or branch when someone tries to change something in the past. This prevents time paradoxes.

4.) We haven't made any receiving devices yet

As others have mentioned, it might be the case that time travel requires receiving devices that we haven't developed yet. It's easy to calculate where the Earth will be 200 years from now, so that won't be a problem, but no device to travel to might be.

5.) There might be infinite timelines for time travelers to go to and we're one of the infinite that don't get traveled to

Maybe time travelers can travel to any of an infinite number of time lines. the chances of one landing on our time line is infinitesimal. There are trillions of years for them to travel through and trillions of universes, but maybe only a few time travelers so far. The odds of them showing up during the last 5,000 years of our recorded history out of the trillions of years that our timeline will exist are very slim.

$\begingroup$I think you are correct with theory #2. Humans are a parasite to this planet and it does not take much of a leap to believe that we destroy ourselves in the next century or two.$\endgroup$
– JeffNov 16 '16 at 14:29

There was a short story by Larry Niven named "Rotating Cylinders and the Possibility of Global Causality Violation", inspired by a mathematics/physics paper of the same name by J. Frank Tipler. In the story, archaeologists investigating the artifacts of an extinct alien civilization have discovered documents describing the theory of these time machines, along with at least one incomplete prototype.

One of the characters theorizes that the universe itself somehow acts to prevent the activation of any such devices, in order to prevent any violation of its physical laws. In the instance of the extinct alien race being investigated, I believe that the sun of their star system had gone nova before they could complete the device.

I am going to address the second option on your list, Many Worlds, in more detail. In my opinion, it is the only physically-sound explanation for how time travel could exist while no effects from it would be visible. That said, Many Worlds is itself a bit of physics-woo, so make of it what you will.

The basic concept of Many Worlds is that for every possible outcome of an event, the universe splits, and both (or however many outcomes are possible) occur.

It is closely related to the concept of Quantum Immortality, which comes out of the application of Many Worlds to Schrodinger's Cat. In that thought experiment, a cat is left in an undermined state inside a box, either alive or dead. The primary issue is that, if the cat is in an undetermined state, what does the cat perceive before the box is opened? Many Worlds suggests the cat experiences both continued life and death; Quantum Immortality is the suggestion the cat would merely experience continued life, thus becoming immortal.

This is related to time travel in that it suggests every possible event, no matter how incredibly unlikely, occurs. Considering phenomena such as zero point energy, where probability is the main factor preventing absurd random occurrences (and entropy, the addressing of which is beyond this answer), the word "possible" can effectively be removed from that statement: every event occurs.

Now, imagine some time traveler returns from 2222 to now. Let's presume the time machine comes with them, for the sake of discussion. What is the probability that a time machine and a time traveler appears in your front yard in the next hour? It's insanely - nearly impossibly - unlikely. But following certain conditions and making certain assumptions, it is possible for that to happen. Meaning in some universe, it must happen.

Now, there is an issue with this, and it is where the cat returns. From your point of view, the probability of that occurrence is so vanishingly small it is easy to ignore. People and time machines do not just materialize out of thin air. But the probability from the time traveler's point of view is substantially higher. If the trip does not fail and he is not killed in the process, his only option is to appear in your front yard.

It's worth pointing out this would suggest probability can be "localized" to a given person. Again, that's well beyond the point I am making.

Because of the difference in the probability of outcomes between you and the time traveler, we end up in a case where the time traveler is effectively routed to a specific branch of time where he and his machine popped into being. You and I never experience this because it is so insanely unlikely that just in a pure probabilistic sense, we end up in some other branch of time. However, at least some number of you in other branches of time do see the time traveler. There's just many more of you that do not experience a time traveler appearing in their yard.

It is also worth pointing out this would mean temporal paradoxes are actually impossible to form, since upon going back in time, the time traveler would be entirely decoupled from their own origins. They effectively just spontaneously came into existence remembering "their" future.

So to summarize all that physics-woo, the Many Worlds interpretation applied to time travel would suggest it is merely very unlikely we end up in the same universe as any given time traveler. It's not impossible, but so insignificantly unlikely we can go about our lives with it never actually happening.

Anyone arriving from 'not now' (either direction) would, by so arriving, automatically create an alternate timeline to 'contain' them. So there may in fact be alternate histories in which people are aware of time travelers. But we are all stuck in the original timeline and never get to see anyone arrive.

[On a side note: nearly every depiction of time travel is impossible because anything which travels, by arriving or leaving, automatically violates conservation of energy. The only depiction which stands a chance of avoiding this is the 'Terminator' kind in which pieces of space-time - presumably of the same energy content (though how is this enforced?) - are exchanged between source and destination]

$\begingroup$Why would we expect time travel to conserve energy? Conservation of energy is just an observation, there's no entity that actually enforces it. Physically, as far as I understand it most explanations rely on time symmetry (see "Why is energy conserved?" on Physics SE) and I don't see why that couldn't be broken as a result of time travel.$\endgroup$
– zetaAug 11 '16 at 3:48

1

$\begingroup$Glad to see this argument used. It's one I have maintained for some time now. As a consequence, time travel would be discontinued. Time travellers would disappear from their home timeline and never be seen again. The original timeline never benefits from time travel. Yet it bears the cost and effort of making time machines and losing the people who become time travellers.$\endgroup$
– a4androidAug 14 '16 at 10:10

1

$\begingroup$About your side-note: conservation of energy applies to closed systems. If time travel can only happen in a closed system, yes conservation of energy will apply. If time travel happens in open system, ones that aren't closed, then conservation of energy not apply. In fact, no more than it does to motion through space. Fantastic answer about time travellers lost in an infinity of alternative histories!$\endgroup$
– a4androidAug 14 '16 at 10:14

$\begingroup$Where in the Terminator franchise do any time-travelling objects or "pieces of space-time" get "exchanged"? I might've just missed it, but it doesn't sound familiar at all. I guess it was called the "time displacement mechanism", but was that ever revealed to actually exchange matter? I always just assumed it meant the user was displaced in time, nothing more elaborate. Knowing what expanded-universe stuff is typically like, there's probably an entire book on this that I've not read, heh.$\endgroup$
– underscore_dAug 15 '16 at 11:55

It's very simple. Time Travel only goes one way. It's called cryogenics and right now it generally has a 100% fatality rate.

The problem with this question is that it makes a faulty assumption. It assumes time exists. A better approach is to ask oneself, "What is time?"

Simply put, time is perception. It's possible to perceive time at an incredible rate. Don't believe me? You're living proof. To something like an idea or concept that stands outside of the universe's perception, the all of reality might happen in an instance. It isn't slow, it just perceives events at a different perception. Another aspect is causality. Humans evolved in a way to create assumptions. We assume a beginning and an end. This instinctively creates a range. Here's another fundamental flaw. The universe isn't a dog or cat. It's timeless and infinite. It's not old, because that would require that it didn't exist at some point. Scientist busy themselves trying desperately to prove a beginning. This is because the other option is infinitely terrifying. The universe is in a way, its own cosmic horror. It defies reason. It is the absents of reason. In short, it's hell.

Time works along the same misconception. If time existed, it would mean that at every point that the universe exists, there is a backup. The problem with this, is that there isn't a place to store something like that. Infinite isn't a number. It's a concept. It's something that isn't big. It just is. Causality is a human construct. If we didn't have memories or our memories were a jumble, time wouldn't be a concept at all.

Instead of thinking of time as something to be traveled, it's more accurate to realize that there is just motion. We build our own time. The universe won't be bent to anything's will. Even something like an elevated idea can't undo an event. When a lightbulb breaks, you can only replace it.

So in short, time doesn't exist. A lot of people are making a whole lot of money creating a comforting lie. The sooner the elites stop looking for comfort the sooner we can get back to fixing real issues.

P.S. I'm sorry if this seems like a downer. I'm not trying to insult anyone. If this is pure science fiction speculation, then I have missed the point entirely and I apologize.

$\begingroup$This is a great post, however it does not answer the question "if time travel is possible in the future, no matter how distant, why havent they come back to tell us?"$\endgroup$
– AricAug 11 '16 at 13:37

$\begingroup$On that note, what if it requires that the device exist on both sides of the jump to work? Maybe you need a machine to jump to?$\endgroup$
– KayotAug 17 '16 at 11:23

1

$\begingroup$That is a valid point, and it is one of the possibilities i am considering.$\endgroup$
– AricAug 17 '16 at 11:25

$\begingroup$I don't get what you mean with the following sentences, can you clarify? "If time existed, it would mean that at every point that the universe exists, there is a backup. The problem with this, is that there isn't a place to store something like that." Specifically, why can't there be a "beginning" of things, and what do you mean with "storing"?$\endgroup$
– BlackBearAug 17 '16 at 19:48

Actually, the very first time traveller started out now, in 2016, going back some twenty years.

When he got there, they asked him who was running for president in 2016. When he tried to tell them it was Hillary Clinton versus Donald Trump, they put him in a quiet room where he couldn't do anybody any harm.

It's too hard to find parking

Time travel requires you to specify a time and a location.
As you would guess, specifying an exact location is quite important. If you appear 100 meters in the air, you die. If you appear 2 meters underground, you die. if you appear on a wall, you die. Would you risk it?

A time machine can't be expected to know what it means to land "in someone's back yard", it needs exact coordinates, with the axis starting from, i don't know, probably the center of the universe or something.

As soon as we start recording exact coordinates of safe landing spaces on earth, people from the future will appear.

$\begingroup$So develop a time machine with a parachute and appear somewhere between 100 meters in the air and a few kilometers in the air. Should be easy enough to fall somewhere in the range where you can gently fall to the ground with a parachute.$\endgroup$
– forestAug 3 '18 at 8:52

$\begingroup$If you can land in a few kilometers' range you're in fantastic shape. But it's an infinitesimally small target to hit when you try to shoot blind$\endgroup$
– DotanAug 4 '18 at 9:29

Answer 1: Why would they bother, what possible motivation would they have for this. They know it is possible and exists they wouldn't need to prove it to the past.

Question 2:

Surely, they would bring that technology back in time to let the
people of the past use it?

Answer 2: See Answer 1

Now lets look at your theoretical explanations for why this might not occur:

I have four theoretical explanations so far, can anyone explain them
in more detail, or provide other possibilities?

Theory 1:

The people decided to ban backwards time travel...

Response: People ban lots of things and still people do those things, so most likely some people would still do that thing.

Theory 2:

The people in the future went back in time to an alternate past.

Responses: Personally I've always viewed alternate pasts as a hack trope of time travel plots. I comprehend alternate futures, but the past seems rather fixed from the POV of whatever when you are. Besides if this were the case then it doesn't particularly matter.

Theory 3:

The people come back in time ... then jump forward in time... they
return to is different to the one they left.

Response: This is basically the classic many worlds/alternate timelines trope. Again if their present changes, it is still our relative future so no big deal from our POV.

Theory 4:

They cannot go back in time ...therefore they can only go forward in
time.

Response: So basically.... they are us. Traveling forward in time and never back.

But have you considered that perhaps they have and do. Maybe they have been here maybe they have marched up the steps of the white house and declared that they are from the future and presented us with a time machine. Maybe they've created countless paradoxes.

Here is the funny thing about time travel.
We do it all the time without thinking about it, albeit in one direction. If we could turn around and go "the other way" we would un-perceive things going in reverse. Funny thing about this is that in-fact there is no reason to believe that we are in fact going in the "right" direction we may live in a back-wards universe with crazy things happing like, gravity pulling things together, light coming out of stars and and people being "born".

Now...if we could jump back or forward in time but then continue forward in time at our normal temporal pace.. ah this would be what we think of as time travel, with that concept, there are basically four primary possibilities I can see.

Time travel is only possible in one direction. Period. (borring)

Time travel is possible (at this point we make an assumption that if time travel is possible it is impossible to avoid all paradoxes)

There is some mechanism outside of the known universe to prevent paradoxes from forming.

The universe is always in a state of paradox but because we are in the paradox we are unaware of it

Any universe in which time travel is possible is a universe that will ultimately destroy itself before it exists.

Personally I like this last option... unfortunately this also means that the first option is also true.

So the point of all this is that. In a universe where time travel is possible there is no reason to believe that we would know it when we saw it even if it were sitting right in front of us.

Most of the ways we have hypothesized to time travel require that the ability to time travel exist in the time you are trying to get back to. In other words, if Time travel is possible, because we haven't invented it yet, they can't get back to us. It's really kinda simple ^.^

There's a law that says it's illegal to travel into the past. Governments fear that even the smallest time travel may alter the present and cause unforeseeable consequences. The time travel technology is in the hands of so few people that it's possible to enforce the law.